E. coli enters the energy game, could pump out petroleum

Bacteria are big news in the biofuel business—genetically engineered microorganisms can turn various combinations of biomass, sunlight, and even carbon dioxide into liquid fuels. The potential for replacing fossil fuels with these renewable energy sources is significant, but the efficient implementation of these technologies remains complicated.

Two studies published in the Proceedings of the National Academy of Sciences this week looked at how to make biofuel production more efficient.

A team of British scientists genetically engineered E. coli bacteria to produce a hydrocarbon molecule that mimics petroleum. Right now, conventional biofuels like ethanol have to be blended with sufficient traditional fuel so that our engines don’t detect their presence. These "mimic molecules," on the other hand, could go straight into our existing engines.

Many hurdles remain to scaling up these engineered bacteria, but the researchers believe it could be a viable alternative to producing ethanol from food crops.

To harvest the fuel molecules produced by a microorganism like E. coli or algae typically requires killing the cell, but new research shows that existing cellular transport mechanisms can be harnessed to excrete biofuel molecules so that cells can continue production over time. “It’s the same idea as milking a cow,” said Geoffrey Chang, a professor of pharmacy at the University of California San Diego.

What brought a pharmacist into biofuel research? Drugs and biofuels are both greasy molecules, so Chang and his colleagues decided to see if they could apply what they knew about the systems that transport pharmaceutical molecules through cell membranes.

Chang and his co-authors, Rupak Doshi and Tuan Nguyen, created a model system using E. coli bacteria that produced molecules called carotenoids. Carotenoids have a similar structure to several biofuel molecules, but they are colorful enough that the researchers could see the transportation in action.

They focused on a common family of transport proteins known as the ATP binding cassette transporters (typically called the ABC transporters), which excrete a variety of greasy molecules, like lipids and steroids.

“The idea is that you can plug and play this transport system. These are ABC transporters, so many organisms have them already,” Chang said.

For many of the microorganisms that have been engineered to produce valuable biofuels, the fuel molecules are toxic. If the cells can pump out the molecules, they can continue to make more without killing themselves.

The E. coli used in the experiment could continue to produce and secrete carotenoids for days. It’s only a model, but because the ABC transporters are common and relatively promiscuous, the approach could be applied in a wide variety of systems. In a practical application, an algae-based system could theoretically continually produce and secrete biofuels indefinitely.

Chang believes that these secretion systems could be a game changer that makes production of biofuels more cost effective. He’s not alone—the leading consumer of jet fuel, the US Air Force, funded the research.

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